Technical Field
This disclosure is directed to apparatus and methods for the alignment of an optical beam with a charged-particle beam within a charged-particle beam microscope, and to the focusing of the optical beam.
BACKGROUND
For more than 400 years, scientists have investigated living organisms and cells with light microscopes. The more recent development of electron microscopy has improved the spatial resolution available to scientists by three orders of magnitude. Most recently, the bridging of these forms of microscopy has involved Correlative Electron and Light Microscopy (CLEM). One approach to CLEM has been to overlap in a charged-particle beam microscope the focused spots of a charged-particle beam, such as an electron beam in a Scanning Electron Microscope (SEM), and an optical beam, such as a focused laser. Once these two beam spots are overlapping in space and focused on the surface of a sample, the sample can be mechanically scanned in a square raster pattern. In this way multiple images that result from the interaction of the optical beam and the electron beam with the sample can be created that are perfectly registered with each other on a pixel-by-pixel basis.
This perfect registration provides an excellent method for comparing the contrast obtained from the optical and electron-beam interactions with the sample. However, the initial alignment of the optical and electron beams is difficult in a charged-particle beam microscope, such as an SEM, because these microscopes are designed to image secondary and backscattered electrons that are produced by the primary electron beam. An apparatus and method is needed to quickly align the optical and electron beams, and focus the optical beam in CLEM applications. In a second example of combined charged-particle and optical spots in a charged-particle beam microscope, in the field of semiconductor manufacturing, it is often necessary during the development of a new chip design to make edits in the prototype circuit to verify the correction of design errors before an investment is made in another set of expensive optical lithography masks. This design-edit function is typically done using a charged-particle microscope to cut traces and deposit new conductive traces with chemical vapor deposition (CVD). CVD deposits have the serious drawback, however, that the material deposited contains impurities having insulating characteristics. The result is that, due to the increased resistivity of the new conductive traces, the modified circuit usually cannot be tested as a standard chip would be tested, and thus the design edit cannot be completely confirmed. It has been found that irradiating and heating such CVD deposits with laser energy will reduce such impurities and thus enable more reliable circuit testing, greatly improving the efficiency of the design-edit function. It is desirable to focus the laser beam so as to target small areas of interest, and also to maximize the optical irradiance on the sample for efficient processing.
Thus, an apparatus and method are also needed to accurately and quickly align a charged-particle beam with a laser beam in a charged-particle beam microscope and focus the beams so that the benefits of applying laser energy to the same location on a chip can be exploited. Further, such alignment should be automated as much as possible to further speed up the alignment process.